Hello moe,
I think most of us have encountered a similar problem. I can also understand the sensitive nature of the issue. There are different ways to approach the subject. Personally I think it depends entirely on the sensitivity of the patient. Depending upon how you think the chap in Q may respond you could advise him that to decrease the possibility of potential infection all patients are advised (by letter, just to him. (I won't tell him hes the only 1 though)

that pts. are advised due to current regulation, to soak their feet prior to their podiatry appt. (He may get the hint & have a good wash?). I think my choice would be the other option & book him in as the last patient of the day. Cowardly I know but if its only the smell thats the issue & you can cope I reckon this may be the more appropriate action. Alternatively is it appropriate to enquire if he has problems self managing with his personal hygeine. I have contacted social services on a few occasions for advice RE: access to services for individuals with difficulties with personal hygeine. Could it be an incontinance issue? Let me know how you get on. :)
Regards